A secondary battery having electric characteristics, such as high ease of applicability according to a product group and high energy density, has been commonly applied to electric vehicles (EV), hybrid electric vehicles (HEV), energy storage systems, or the like which are driven by an electric driving source, as well as portable devices. The secondary battery has received public attention as a new energy source for promoting eco-friendly performance and energy efficiency in that byproducts are not generated at all according to the use of the energy as well as a primary advantage of dramatically reducing the use of fossil fuels.
The secondary battery is repeatedly chargeable and dischargeable by an electrochemical reaction between constituent elements including positive and negative current collectors, a separator, an active material, an electrolyte, and the like. A lithium polymer secondary battery which is widely used as an example has an operation voltage of about 3.7 V to 4.2 V. Accordingly, the battery pack is configured by connecting a plurality of unit secondary battery cells in series in order to obtain a high-output battery pack which is applied to the electric vehicles and the like.
In addition to the basic structure, the battery pack may be configured to additionally include an overvoltage protection circuit that cuts off the charge when an overvoltage situation occurs by measuring the voltage of the battery pack in order to prevent the overvoltage charge during charging. However, in order to cut off the charge, since the voltage of the battery pack needs to be continuously sensed, the cut-off circuit has no choice but to operate normally, and accordingly, power consumption cannot help being large. Further, even in a situation where the battery pack is not used, since the cut-off circuit operates, there is a problem in that the battery pack discharges quickly.